The present invention relates to aqueous aminoplast resins based on melamine of urea and formaldehyde, that upon curing are characterized by low formaldehyde emission, to processes for preparing the resins, and to particleboard and other composites made from the resins.
Urea-formaldehyde condensation products are widely used in industry. One important use of these condensation products is as adhesives and binders in the manufacture of particleboard.
Urea formaldehyde (UF) resins have the virtues of low cost, rapid cure, processing convenience, and clear color. Very short press cycles can be achieved with urea formaldehyde adhesives; by adding a catalyst, the rate of cure can be adjusted to essentially any desired speed.
However, whenever an amino-formaldehyde adhesive is used in the production of a structural panel, by whatever process, the generation of formaldehyde fumes remains a problem.
Formaldehyde release is especially noticeable in particleboard and in insulation foams. Both of those contain cured resin films with very large surface areas which enhance formaldehyde release. The causes for formaldehyde release are complex. From the resin itself, free, unreacted formaldehyde may evolve. Also, formaldehyde dissolves in moisture in wood products, and its vapor pressure and its release rate change with changes in air humidity and product humidity. In particleboard, released formaldehyde can come from formaldehyde which was bound to wood cellulose during the hot-press cycle, and which slowly hydrolyzes under the influence of the acidic humidity in the wood. It can also result from the degradation of incompletely cured resin, or resin components, such as methylolurea. Finally, it can result from bulk resin degradation.
The UF resins contain methylol, methylene ether and other reaction products which can hydrolyze back to formaldehyde. The weakest links are in the cellulose-resin link, the hemiacetals, ethers and methylols. The oxygen-free methylene linkage is the most resistant to hydrolysis.
Several paths have been explored over the last few years for reducing formaldehyde release. These include coating applications, chemical treatments before or after resin application, the use of resin additives, and new resin formulations. In such new resin formulations, the mole ratio of formaldehyde to urea has been slowly decreased over the years from its initial high value, but reduction in this ratio generally weakens the internal bond in particleboard, for example, even though it reduces the residual formaldehyde. Some resin manufacturing operations are now programming formaldehyde (F) and urea (U) additions in two stages, to achieve a desired low F/U molar ratio.
One generally accepted two-stage procedure for making urea formaldehyde resins involves the reaction of urea and formaldehyde under alkaline conditions to form methylol ureas, followed by resinifying by further heating under acidic conditions, and finally neutralizing and dehydrating to produce a product of the desired physical characteristics. This procedure requires very accurate control of the pH in the different stages of the process to prevent gelation, and it is at times difficult to obtain consistent physical properties.
Urea and formaldehyde will also react under various conditions of controlled acidity, but gelling systems are usually obtained. For example, if a mixture of one mole of urea and two moles of formaldehyde is maintained under acidic conditions, the mass gels unless steps are taken to interrupt the course of the reaction by the adjustment of the pH at the appropriate times.
In the two-stage, alkaline then acid reaction used for the commercial manufacture of urea-formaldehyde prepolymers, for use in adhesives, the prepolymer resins are made by preparing a urea-formaldehyde solution having a F/U molar ratio ranging from 1.5 to 2.5. This solution is made basic with sodium hydroxide, triethanolamine, triethylamine, ammonia or any appropriate base that will establish a pH in the range of 7.5-8.9.
This basic solution is then brought to reflux for approximately 15-30 minutes, cooled slightly, and the pH is adjusted to a range of approximately 5.5-6.9 using formic acid, p-toluenesulfonic acid or other appropriate organic or inorganic acid. The acidic solution is then brought to reflux until a specific Gardner viscosity has been reached. At this predetermined viscosity point the temperature is dropped slightly, the resin adjusted to a pH of 7.2-7.6, and additional urea is added as required. Water is then removed under vacuum until a desired specific gravity is obtained or a desired percent solids reached. The resin is then cooled and ready for shipment as a prepolymer prior to final cure by the addition of acid.
This prepolymer commercial resin usually has a free formaldehyde content in the range of 0.5%-1.8%, but depending on the resin and its intended application, the free formaldehyde content may be as high as 5%. This two-stage manufacturing procedure results in a prepolymer resin containing methylol, dimethylene ether, and methylenediurea groupings.
The reactions which occur in such processes are of two kinds. Under the mildly alkaline conditions used in the first stage, both monomethylolureas and dimethylolureas are formed. The second stage involves the condensation of methylolureas, under acidic conditions, to form dimethylene ether bridges. Reactions can also occur between the methylol group and the amido hydrogen of urea to form methylene bridges. The subsequent polymerization of cure of such prepolymers normally goes through two distinct stages. The first stage of curve involves the formation of a low molecular weight fusible, soluble resin. The second stage of cure involves a reaction which converts the low molecular weight urea-formaldehyde resin into a high molecular weight network polymer. Cure is usually accomplished by heating under acidic conditions.
It has been postulated that various ether linkages in uncured resins further aggravate hydrolytic degradation in the cured state. There is also a large body of literature on the acid hydrolysis of compounds, having similar structures to that associated with the cured resin, which demonstrates that the different linkages which may exist in cured UF resins could possess wide variations in hydrolytic stability. The following crude order of relative hydrolytic stabilities for possible links in a crude UF network has been postulated: methylene bridge&gt;dimethylene ether bridge&gt;methylol end group.
Many variations U-F condensation techniques have been tried, but there remains a need for a urea-formaldehyde resin which, when cured, will be characterized by low emission of formaldehyde, and will have equal, if not better, adhesive and bonding properties than those associated with current urea-formaldehyde resins.
My copending application Ser. No. 416,573, filed Sept. 10, 1982, now U.S. Pat. No. 4,410,685, discloses a new resin formulation which is hydrolytically stable. The new resin is a urea-formaldehyde liquid base resin comprising urea and formaldehyde in a mole ratio of substantially 1:1. This resin contains essentially no free formaldehyde. When cured, it contains substantially more methylene groups than methylene ether groups. My application also discloses a process for preparing this resin, comprising adjusting a formaldehyde solution containing from about 49.8% to about 50.2% formaldehyde to a pH of 0.5-2.5, slowly charging urea to said formaldehyde solution while maintaining the temperature at 40.degree.-70.degree. C. and the pH at 0.5-2.5, showing the addition of urea to the reaction mixture after attaining a Gardner viscosity in the range of A-D, neutralizing the reaction mixture after obtaining a Gardner viscosity in the range of U-V+, then adding the final charge of urea and permitting equilibration.
A second copending application of mine, Ser. No. 416,574, also filed Sept. 10, 1982, now U.S. Pat. No. 4,409,293, discloses particleboard made from the new resin, and processes of using the resin as a particleboard binder. Both of these applications are expressly incorporated herein by reference.